Coating compositions are already known, from DE-A-40 24 204, which as binder contain a hydroxyl-containing polyacrylate resin which has been prepared in the presence of a hydroxyl-containing polyester. In particular if these coating compositions are applied at low temperatures, the masking resistance and the solvent resistance of the resulting coatings are, however, in need of improvement.
Coating compositions based on hydroxyl-containing polyacrylate resins and crosslinking agents are known, for example, from JP-A 4-1254. In this document, the hydroxyl-containing polyacrylate resins employed as binders are obtainable from hydroxyl-containing monomers, alkyl acrylates, alkyl methacrylates, optionally styrene and optionally ethylenically unsaturated polymers. In this case it is essential to the invention that the polyacrylate resin has been prepared using 4-tert-butylcyclohexyl acrylate and/or 4-tert-butylcyclohexyl methacrylate as monomer component. Hydroxyethyl acrylate and/or hydroxyethyl methacrylate, in particular, is employed as hydroxyl-containing monomer.
These coating compositions known from JP-A 4-1254, especially when used as transparent topcoat over a basecoat film, have the particular disadvantage that, when hydroxyethyl acrylate and/or hydroxyethyl methacrylate is used as OH monomer, the resulting coatings are of inadequate adhesion to the basecoat film. A further disadvantage of these coating compositions known from JP-A 4-1254 is the inadequate pot life of the coating compositions.
EP-B-0 158 161 discloses hydroxyl-containing acrylate copolymers produced by copolymerization of acrylate monomers having at least two olefinically unsaturated double bonds, hydroxyl-containing monomers and further olefinically unsaturated monomers, which together with melamine-formaldehyde resin as crosslinking agent can be employed as coating compositions.
A feature of these coating compositions known from EP-B-0 158 161 as well is that, especially when used as transparent topcoat over a basecoat film, they exhibit the disadvantage of resulting, when hydroxyethyl(meth)acrylate is used as hydroxyl-containing monomer, in coatings having inadequate adhesion to the basecoat film, and that the pot life of the coating compositions is inadequate.
EP-A-0 638 591 describes hydroxyl-containing copolymers which can be prepared by a bulk polymerization in which one component is included in the initial charge at the beginning of the polymerization and, in the subsequent course of the polymerization, at least two olefinically unsaturated monomers are added, of which at least one contains at least one carboxyl group and at least one is sterically hindered. Also described is the use of such copolymers in high-solids coating compositions. Again, the adhesion of such coating compositions in accordance with EP-A 0 638 591 to basecoat films is inadequate.
DE-A 44 07 415 discloses coating compositions which provide an outstanding solution to the problems resulting from the prior art to date. Such coating compositions include (A) at least one hydroxyl-containing polyacrylate resin, obtainable from (a) a cycloaliphatic ester of (meth)acrylic acid, (b) a hydroxyl-containing ester of (meth)acrylic acid, (c) optionally a hydroxyl-containing, ethylenically unsaturated monomer which is different from (a) and (b), (d) an aliphatic ester of (meth)acrylic acid which is different from (a), (b) and (c), (e) optionally a vinyl-aromatic hydrocarbon which is different from (a), (b), (c) and (d), and (f) optionally a further ethylenically unsaturated monomer which is different from (a), (b), (c), (d) and (e), and (B) at least one crosslinking agent.
When used as transparent topcoat over a pigmented basecoat film, the coating compositions according to DE-A-44-07 415 exhibit markedly improved adhesion to the basecoat film coupled with good chemical resistance and good weathering resistance. The coating compositions can be cured at low temperatures and can therefore be employed for the sector of automotive refinishing, Even when the coating compositions are cured at these low temperatures, the coating compositions rapidly cure fully but at the same time exhibit processability for a sufficiently long period. Moreover, the coating compositions according to DE-A-44 07 415 have the advantage of a good topcoat holdout, exhibit very good flow and good application properties under the conditions of automotive refinishing, and, when used as clearcoat over a basecoat film, they do not start to dissolve this film to any great extent. Recent requirements of the market necessitate, however, a level of adhesion which is improved again over the coating compositions according to DE-A-44 07 415 when the coating compositions are employed as clearcoat for the coating of basecoat films, and necessitate a further-increased gasoline resistance and improved polishability.